1. Field of the Invention
The present invention relates to servovalves that are used to transfer quantities of fluid. The rotary servovalve and control system of the invention is particularly useful in situations requiring rapid response and precision control of fluid flow.
2. Description of the Prior Art
Many different mechanical and electrical servovalve systems have been employed for controlling fluid flow in industrial and manufacturing environments, as well as in other applications. Fluid servovalve control systems which require precise and highly responsive control are employed in a multitude of widely varying applications, including the control of robots, the operations of presses for manufacturing rubber and plastic parts, the control of tensioning devices in the paper industry, automotive vehicle and parts manufacturing, petroleum refining operations, and numerous other applications.
In many of the applications in which servo control valve systems are utilized, pressurized fluid is typically provided from a high pressure source and transmitted through a load from which the fluid is then exhausted to a low pressure reservoir. The load may, for example, take the form of a double-acting piston operating within a cylinder. The transfer of fluid from one side of the piston to the other within the cylinder causes the piston to move some mechanism to which it is connected.
Servovalves are widely utilized to control the flow of both pneumatic and hydraulic fluids. Conventional servovalves are often linear motion devices. That is, they typically consist of a spool element operated manually or electrically. Typically the spool element is shuttled back and forth within a chamber defined within a valve housing. By its movement the spool element covers and uncovers different fluid orifices or ports.
The function of a servovalve is to control the velocity and quantity of fluid flow and the direction of movement of a piston within a cylinder. Conventional servovalves typically contain a high torque motor connected to a flapper assembly. Such servovalve systems are expensive and require high manufacturing tolerances. They are also sensitive to contamination and clogging. Furthermore, they typically require continuous flow from a pump in order to operate.
Efforts have been made to develop rotary servovalves for use in servovalve systems. One such rotary servovalve is described in U.S. Pat. No. 4,794,845 which describes the application of a torque motor to a servovalve system. The torque motor controls flow by rotating a spool element within a complex sleeve assembly containing appropriate fluid passageways. In the event of a power failure a rather intricate arrangement of mechanical elements, including torque rods and springs, is required to center the servo valve in order to halt fluid flow. Also, when power is available, energy is required to center the valve for a zero command, no flow condition. This is necessary because torque motors are directional devices, but have no detent or zero position when power is removed. Consequently, the torque motor must always be energized or actuated throughout operation of a conventional rotary servovalve system.
U.S. Pat. No. 5,597,014 is directed to a high flow, direct drive rotary servovalve. This patent describes improvements in passageway designs, the function of which is to eliminate tangential Bernoulli reaction forces acting on the spool member. These forces tend to close the valve, thus requiring the torque motor to expend more energy in order to keep the valve open.
Prior rotary servovalve and control systems lack the requisite torque to be of commercial utility in many applications. This is because of the Bernoulli forces that are created with fluid flow and which tend to close the valve. Prior rotary servovalve and control systems provide elaborate means to combat the influence of these Bernoulli forces. For example, the rotary servovalve of U.S. Pat. No. 4,794,845 employs a very complex arrangement to overcome the Bernoulli forces.